The present invention relates generally to laser trimming and more specifically to laser trimming above a dielectrically isolated single crystal region.
Laser trimming of thin-film resistors is used extensively to produce improved accuracy in analog integrated circuit technology. In integrating laser trimming into dielectrically isolated circuit technology, a difficulty peculiar to dielectric isolation has been identified. Trimming is generally accomplished by use of an infrared laser for improved control. Silicon is nearly transparent at the wavelengths used; this results in laser energy penetrating to the bottom of the dielectrically isolated island, reflecting back and transferring some of the reflected energy back to the resistor. The result is poor control due to interference effects. These effects are variable due to changes in dielectrically isolated island depth and proper control over trim energy hence becomes very difficult.
An existing technique addresses the problem by simply placing the thin-film resistor over the polycrystalline silicon used to support the dielectrically isolation regions. The polysilicon is much thicker than the single-crystal islands (typically 10 mils vs. 1 mil) and energy scattering off the grain boundaries soon dissipates the laser beam. The resulting lack of reflection and interference produces enhanced controllability. The resistor is deposited over a polycrystalline surface which is not perfectly flat but in fact possesses considerable relief at the grain boundaries. In addition, large polycrystalline areas tend to "dish out" during the grind and polish operation which complicates laser focusing and reduces photoresist definition, resulting in poor control over resistor geometries.
One solution is suggested in corresponding U.S. patent application Ser. No. 518,725 filed July 29, 1983 now U.S. Pat. No. 4,468,414 N. W. Van Vonno. An opening is provided in the dielectric isolation at the bottom of the single crystal island exposing the polycrystalline support. Thus, infrared radiation passes into the support and does not reflect back to the top surface of the single crystal island. In some applications, all islands must be dielectrically isolated.
An object of the present invention is to provide a method of fabricating laser trimmed thin film resistors over single crystal islands.
Another object of the present invention is to provide a method of laser trimming thin film resistors on dielectrically isolated integrated circuits.
Still another object of the present invention is to provide a method for fabricating dielectrically isolated islands with non-reflective bottom surfaces to be used with subsequent laser trimming steps.
A further object of the present invention is to provide an integrated circuit having laser trimmed thin film resistors on dielectrically isolated integrated circuits.
An even further object of the present invention is to provide a totally dielectrically isolated integrated circuit having improved laser trimmed thin film resistors thereon.
These and other objects of the invention are attained by fabricating dielectrically isolated islands with a substantially non-reflecting or poorly reflecting bottom. The thin film resistors are formed over an insulative surface on the single crystal surface and are subsequently laser trimmed. The bottom of the dielectrically isolated island including a polycrystalline support structure may be rendered non-reflective by forming a plurality of indentures in the bottom of the island prior to the growth of the insulating oxide. The indentures may be formed by etching a parallel pattern of shallow V-shaped grooves. Alternatively, the surface may be rendered non-reflective by etching with an appropriate etchant to produce a rough surface on the bottom of the island. Yet another method involves rendering the island bottom non-reflective by mechanically damaging or abrading the silicon surface by such well-known means as sandblasting and mechanical abrasion. All of the above alternative methods are followed by growing of the dielectric isolation layer over the bottom of the substrate and face of the moats, applying the polycrystalline support structure and removing most of the original substrate to form the dielectrically isolated regions. The non-reflective, indentured regions may be a portion of the total dielectric isolation layer and the thin film resistive material is applied and delineated on the top of the dielectrically isolated region to form a resistor juxtaposed to the nonreflective bottom dielectric isolation. This is followed by laser trimming using a laser of suitable wavelength and power level.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.